Under certain operating conditions, engines that have high compression ratios, or are boosted to increase specific output, may be prone to low speed abnormal combustion events, such as due to pre-ignition. The early abnormal combustion due to pre-ignition can cause very high in-cylinder pressures, and can result in combustion pressure waves similar to combustion knock, but with larger intensity. Such abnormal combustion events can cause rapid engine degradation. Accordingly, strategies have been developed for early detection and mitigation of abnormal combustion events based on engine operating conditions.
One example approach is illustrated by Shishime et al in US 20110239986. Therein, in response to an indication of pre-ignition and further based on an engine speed at which the indication was received, an engine controller is configured to adjust a fuel injection amount and timing to enrich the affected cylinder and optionally reduce the effective compression ratio. In another example, illustrated by Makino et al. in U.S. Pat. No. 8,731,799, an intake cam is advanced to vary the intake valve timing and reduce the effective compression ratio of the engine. In still other cases, wastegate and/or throttle adjustments may be used to vary the effective compression ratio of the engine. Specifically, the intake airflow and thereby the engine load is reduced. In both cases, the resulting drop in effective compression ratio addresses the pre-ignition by decreased compression causing a decreased temperature rise.
However, the inventors herein have identified potential issues with such approaches. The adjustments that reduce the compression ratio may affect engine performance adversely. As an example, the fuel injection enrichment may degrade fuel economy, degrade exhaust emissions, and result in possible torque reduction is the richness is richer than RBT. Cam timing adjustments may also result in loss of fuel economy. As another example, the advance in intake cam timing may result in residual effects that eventually further exacerbate pre-ignition by increasing residuals.
To address the above-mentioned issues, the inventors herein have developed a method for mitigating pre-ignition in an engine comprising: in response to an indication of pre-ignition, adjusting a piston displacement to reduce an engine compression ratio. In this way, abnormal combustion due to pre-ignition may be addressed by taking advantage of variable piston displacement while fueling and valve timing is maintained.
As an example, a vehicle may be configured with a variable compression ratio engine. Specifically, each cylinder of the engine may include a piston coupled to a piston displacement changing mechanism that moves the pistons closer to or further from the cylinder head, thus changing the size of the combustion chambers. By changing the size of the piston displacement, the static compression ratio of the engine (that is, a volume of the cylinder when the piston is at Bottom Dead Center relative to the volume of the cylinder when the piston is at Top Dead Center) may be varied. In one example, the piston connecting rod may be coupled to a hinged block or an eccentric shaft such that a displacement of the piston within the cylinder can be adjusted. In another example, an eccentric may be coupled to a piston pin, the eccentric changing the displacement of the piston within the combustion chamber. Movement of the eccentric may be controlled by oil passages in the rod. It will be appreciated that still other mechanisms that mechanically alter the displacement of the piston within the combustion chamber may be used without departing from the scope of this invention. By adjusting the displacement of the piston, an effective (static) compression ratio of the engine can be varied. During nominal engine operating conditions, the engine may be operated with a piston displacement that provides a nominal compression ratio. Based on the pre-ignition history of the engine (that is, before an indication of pre-ignition is received), the piston displacement may be reduced to lower the compression ratio to a feedback level. By adjusting the piston displacement to reduce the compression ratio in a feedback manner responsive to pre-ignition history, the engine's propensity for pre-ignition may be lowered. In response to an actual pre-ignition event (for example, an event occurring even after the compression ratio is lowered to the feedback level), the compression ratio of the engine may be immediately further reduced by decreasing the displacement of the piston. The reduction in compression ratio responsive to the pre-ignition event may lower the compression ratio to a mitigation level that is lower than the feedback level. By immediately reducing the compression ratio of the engine responsive to pre-ignition incidence, further abnormal cylinder combustion events may be reduced. Specifically, the reduced compression may reduce the thermodynamic rise of temperature due to a lower pressure rise from reduced compression stroke piston displacement. At the same time, fuel injection amount and timing may be maintained while a cylinder combustion air-fuel ratio is held at or around stoichiometry. Likewise, intake valve timing may also be maintained. The amount of compression ratio reduction applied may be based on the indication of pre-ignition. For example, as a knock sensor output exceeds a pre-ignition threshold and/or as a pre-ignition count or pre-ignition frequency of the engine exceeds a threshold, the piston displacement may be reduced until a threshold compression ratio is reached. Below the threshold compression ratio, engine performance may be affected. Therefore, once the threshold compression ratio is reached, further pre-ignition may be addressed by enriching the engine (e.g., enriching only the affected cylinder) and/or varying valve timing.
In still further instances, the piston displacement induced reduction in compression ratio may be based on the engine speed at which the pre-ignition occurs. For example, when pre-ignition occurs at higher engine speeds, or during transient conditions, piston displacement may not be able to reduce the compression ratio rapidly enough. During such conditions, at least some cylinder enrichment may be applied before the compression ratio is reduced via piston displacement. Following pre-ignition mitigation, as a duration of engine operation with no pre-ignition increases, the engine enrichment and/or load limiting may be reduced to return the engine operation to stoichiometry with no load limiting. Thereafter, in response to no further pre-ignition, the compression ratio of the engine may be returned to the nominal value by gradually increasing piston displacement.
In this way, abnormal cylinder combustion due to pre-ignition may be addressed by varying piston displacement and without changing fuel and valve settings. By reducing the compression ratio of the engine responsive to pre-ignition by rapidly reducing the piston displacement, pre-ignition may be mitigated without relying only on enrichment and load limiting, thereby improving fuel economy and engine performance even while the pre-ignition is addressed. By holding the lower compression ratio for a subsequent duration or distance of vehicle travel until no further incident of pre-ignition occurs, engine degradation due to pre-ignition can be reduced and engine life can be improved. By subsequently returning the compression ratio to a nominal value as pre-ignition incidence drops, engine performance issues resulting from a transient decrease in compression ratio can be reduced. In addition, fuel economy is increased while exhaust emissions are reduced. By reducing the risk of further pre-ignition, unwanted NVH issues associated with pre-ignition events are also reduced.
The above discussion includes recognitions made by the inventors and not admitted to be generally known. Thus, it should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.